I. Cancer:- Cancer results from a series of molecular events that fundamentally alter the normal properties of cells. In cancer cells the normal control systems that prevent cell overgrowth and the invasion of other tissues are disabled. These altered cells divide and grow in the presence of signals that normally inhibit cell growth; therefore, they no longer require special signals to induce cell growth and division.
As these cells grow they develop new characteristics, including changes in cell structure, decreased cell adhesion, and production of new enzymes.
II. Cancer Genetics:- Approximately 35,000 genes in the human genome have been associated with cancer. Alterations in the same gene often are associated with different forms of cancer. These malfunctioning genes can be broadly classified into three groups:-
The 1st group, called proto-oncogenes, produces protein products that normally enhance cell division or inhibit normal cell death. The mutated forms of these genes are called oncogenes.
The 2nd group, called tumor suppressors, makes proteins that normally prevent cell division or cause cell death.
The 3rd group contains DNA repair genes, which help prevent mutations that lead to cancer.
Mutations that produce oncogenes accelerate growth while those that affect tumor suppressors prevent the normal inhibition of growth. In either case, uncontrolled cell growth occurs.
III. Causes of Cancer:- The prevailing model for cancer development is that mutations in genes for tumor suppressors and oncogenes lead to cancer. However, some scientists challenge this view as too simple, arguing that it fails to explain the genetic diversity among cells within a single tumor and does not adequately explain many chromosomal aberrations typical of cancer cells.
An alternate model suggests that there are “master genes” controlling cell division. A mutation in a master gene leads to abnormal replication of chromosomes, causing whole sections of chromosomes to be missing or duplicated. This leads to a change in gene dosage, so cells produce too little or too much of a specific protein. If the chromosomal aberrations affect the amount of one or more proteins controlling the cell cycle, such as growth factors or tumor suppressors, the result may be cancer. There may be multiple mechanisms leading to the development of cancer.
IV. Tumor Biology:- Cancer cells behave as independent cells, growing without control to form tumors. Tumors grow in a series of steps.
The 1st step is a hyperplasia, meaning that there are too many cells resulting from uncontrolled cell division. These cells appear normal, but changes have occurred that result in some loss of control of growth.
The 2nd step is dysplasia, resulting from further growth, accompanied by abnormal changes to the cells.
The 3rd step requires additional changes, which result in cells that are even more abnormal and can now spread over a wider area of tissue. These cells begin to lose their original function; such cells are called anaplastic.
V. Types of Tumor:-
a) Carcinomas result from altered epithelial cells, which cover the surface of our skin and internal organs. Most cancers are carcinomas.
b) Sarcomas result from changes in muscle, bone, fat, or connective tissue.
c) Leukemia results from malignant white blood cells.
d) Lymphoma is a cancer of the lymphatic system cells that derive from bone marrow.
e) Myelomas are cancers of specialized white blood cells that make antibodies.
VI. Viruses & Cancer:- Many viruses infect humans but only a few viruses are known to promote human cancer. These include both DNA viruses and retroviruses, a type of RNA virus.
Viruses associated with cancer include human papillomavirus (genital carcinomas), hepatitis B (liver carcinoma), Epstein-Barr virus (Burkitt’s lymphoma and nasopharyngeal carcinoma), human T-cell leukemia virus (T-cell lymphoma); and, probably, a herpes virus called KSHV (Kaposi’s sarcoma and some B cell lymphomas).
The viral form of this gene is an oncogene called v-SIS. Cells infected with the virus that has v-SIS overproduce the growth factor, leading to high levels of cell growth and possible tumor cells.
VII. Detection & Diagnosis:- Techniques for detecting cancer are imaging techniques such as MRI, X-rays (such as mammograms), CT, and ultrasound, which can provide an image of a tumor. Endoscopy allows a physician to insert a lighted instrument to look for tumors in organs such as the stomach, colon, and lungs.
Most of these techniques are used to detect visible tumors, which must then be removed by biopsy and examined microscopically by a pathologist. The pathologist looks for abnormalities in the cells in terms of their shape, size, and structure, especially the nucleus.
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